Conventional holographic memory systems normally employ a page-oriented storage approach. An input device such as a spatial light modulator (“SLM”) presents recording data in the form of a two dimensional array (referred to as a page), while a photodetector such as a charge coupled device (“CCD”) camera or a CMOS detector is used to retrieve the recorded data page upon readout. Other architectures have also been proposed wherein a bit-by-bit approach is employed in lieu of the page-oriented approach.
Referring to FIG. 1, there is illustrated a conventional page-oriented holographic memory system. The conventional page-oriented holographic memory system includes a light source 100; a beam splitter 102; a reflection mirror 104; a first and a second shutter 105 and 106; a SLM 107; a first and a second lens 108 and 110; a photodetector 112; a spindle motor 120; and a holographic storage medium 130. The photodetector 112 is a charge coupled device (CCD) camera or a CMOS detector.
The conventional page-oriented holographic memory system shown in FIG. 1 records or retrieves data in the following way. The light source 100 emits a laser beam. The beam splitter 102 separates the emitted laser beam into a reference beam and a signal beam. The signal beam is transmitted through the beam splitter 102 to be directed toward the holographic storage medium 130 and the reference beam is reflected by the beam splitter 102 be directed toward the reflecting mirror 104. The reference beam is then reflected by the reflection mirror 104 to the holographic storage medium 130 at a predetermined incident angle.
In case of recording data in the holographic storage medium 130, both the first and the second shutter 105 and 106 are opened so that the signal beam can be transferred to the spatial light modulator 107 and the reference beam can be transferred to the holographic storage medium 130. The spatial light modulator 107 modulates the signal beam into binary pixel data on a page basis. The modulated signal beam is transferred to the holographic storage medium 130 via the first lens 108. The holographic storage medium 130 stores data by storing an interference pattern acquired from an interference phenomenon between the reflected reference beam and the modulated signal beam.
On the other hand, in case of retrieving the data recorded in the holographic storage medium 130, only the first shutter 105 is opened so that the reflected reference beam can be transferred to the holographic storage medium 130 while the second shutter 106 is closed. When the reference beam is irradiated onto the medium 130 in order to reconstruct the data recorded thereon, the reference beam is diffracted by the interference pattern in the medium 130 so that a signal beam is reconstructed. The reconstructed signal beam is detected by the photodetector 112 via the second lens 110 and the photodetector 112 converts the reconstructed signal beam to an electric signal.
However, in case the retrieving process is not performed under a complete dark condition, white light from the sun or an illumination device may be introduced into the photodetector during the retrieving process. The white light is a major cause of noise so that a signal to noise (S/N) ratio of the retrieved data is decreased. Therefore, the holographic memory system requires a photodetecting device capable of substantially preventing the white light from being introduced thereinto while detecting the reconstructed signal beam.